Heart and kidney transplants will be continued between highly defined inbred strains of mice. The mechanism by which H-2 incompatible kidney transplants and, to a lesser degree, heart transplants survive will be investigated by in vivo and in vitro techniques. A chemically induced sarcoma indigenous to the standard donor strain used in these experiments will also be employed in certain experiments in which the effects of the tumor on recipient immune responsiveness will be compared to those induced by organ transplants.